Mod2: Cardiac Physiology

Question 1

_{ACTION POTENTIALS}

An abrupt pulse-like change or impulse in the membrane potential that is propagated (goes out all ways to all tissues) is known as:

Answer 1

An action potential

Question 2

_{ACTION POTENTIALS}

Which factors could elicit an An action potential (AP)?

Answer 2

Electrical, Mechanical, or Chemical factors that suddenly increases the permeability of the membrane to Na ions

Flicking the heart or giving epinephrine can cause an AP

Question 3

_{ACTION POTENTIALS}

An action potential results from the rapid change in membrane permeability to which ions?

Answer 3

Na+, K+, Ca²⁺, and Cl-

Question 4

_{ACTION POTENTIALS}

What are the three phases of an action potential?

Answer 4

The RMP

Depolarization

Repolarization

Question 5

_{ACTION POTENTIALS}

At which voltage is the Resting Membrane Potential (RMP) said to be polarized?

Answer 5

at -90mv

Question 6

_{ACTION POTENTIALS}

Which processes are reponsible for maintaining the RMP at -90 mV?

Answer 6

Primarily Diffusion and

Sodium potassium pump through active transport

• The membrane is said to be “polarized” as a large negative membrane is present (-90 mV)​*
• RMP is 100x more permeable to K​*

Question 7

_{ACTION POTENTIALS}

What’s another name for what occurs when a stimulus causes the membrane to become more permeable to sodium?

Answer 7

Depolarization

Na enters the cell and causes the potential to increase to -60 mV

_{A stimulus causes the membrane to become more permeable to Na and the potential begins to increase to threshold ( roughly -60 but references say different numbers)}

Question 8

_{ACTION POTENTIALS}

T/F: Threshold is an “all-or-nothing” phenomenom that causes an action potential

Answer 8

True

If Threshold is not reached, the action potential will not happen

Picture shows two stimuli attempted to initiate AP but failed to reach threshold

Question 9

_{ACTION POTENTIALS}

What happens once the membrane reaches threshold?

Answer 9

Once threshold is reached, an action potential ensues and the Sodium channels open allowing for Na to come into the cell (depolarization)

More sodium channels open until membrane potential is +35 mV

Question 10

_{ACTION POTENTIALS}

What happens during repolarization?

Answer 10

Na permeability stops at about +35

Membrane becomes permeable to K+, which rushes out

This causes a decreased permeability to Na

Membrane potential return back to -90mV (RMP)

Repolarization has occured

Question 11

_{ACTION POTENTIALS}

What are the two types of action potentials?

Answer 11

Fast Action potentials

Slow actions potentials

Question 12

_{ACTION POTENTIALS}

Which Action Potentials are mediated by the opening of a large number of Na channels?

Answer 12

Fast Action potentials

Question 13

_{ACTION POTENTIALS}

Why are Fast Action potentials termed “open”?

Answer 13

Because they remain open for only a few thousandths of a second and then abruptly close

At the end of the closure, repolarization occurs and the AP is over until another one comes

If threshold is not reached, another action potential will not occur

Must be stimulated by an action potential to depolarize

Mediated by Na

Question 14

_{ACTION POTENTIALS}

Where do Fast Action potentials occur?

Answer 14

Located in atria, ventricles, and purkinje system

Question 15

_{ACTION POTENTIALS}

Which action potentials are are slower to open and close, can spontaneously depolarize

Answer 15

Slow Response Action Potentials

Question 16

_{ACTION POTENTIALS}

Where do Slow actions potentials occur?

Answer 16

Pacemaker cells SA node ( norm) & AV node

Under abnormal circumstances, the bundle of His, purkinje system, or cardiac muscle can take over

Question 17

_{ACTION POTENTIALS}

T/F: Slow Action Potentials Do not have a true RMP, whereas Fast Action Potentials do.

Answer 17

True

Question 18

_{FAST RESPONSE ACTION POTENTIAL(SODIUM MEDIATED)}

Describe Phase 0 of the Fast Response or Sodium Mediated Action Potential:

Answer 18

Phase 0: Rapid depolarization (Na in)

Corresponds to QRS

Occurs because Na permeability increases 100 fold

RMP increases from -90 to about -40 to -65 (depending on resource use) where threshold is reached

Once threshold is reached, AP ensues, and a second channel opens, which is calcium

At the onset of the action potential, K= permeability decreases 5 fold, delaying repolarization

Another AP cannot come in and cause depolarization until Repolarization occurs

Question 19

_{FAST RESPONSE ACTION POTENTIAL(SODIUM MEDIATED)}

Describe Phase 1 of the Fast Response or Sodium Mediated Action Potential:

Answer 19

Phase 1: Overshoot

This is the tip of the peak in which the cell membrane remains relatively impermeable to K

Inactivated H gates close and stop the influx of sodium

Question 20

_{FAST RESPONSE ACTION POTENTIAL(SODIUM MEDIATED)}

Describe Phase 2 of the Fast Response or Sodium Mediated Action Potential:

Answer 20

Phase 2: Plateau phase

Corresponds to the Q-T interval

There are three causes to the Plateau phase

Question 21

_{FAST RESPONSE ACTION POTENTIAL(SODIUM MEDIATED)}

What are the three causes to the Plateau phase (Phase 2)?

Answer 21

1. Calcium channels are open fully (sodium and calcium are entering)
2. Delayed closure of some of the sodium channels
3. Permeability of potassium decreases even more (another 5-fold)

Question 22

_{FAST RESPONSE ACTION POTENTIAL(SODIUM MEDIATED)}

Describe Phase 3 of the Fast Response or Sodium Mediated Action Potential:

Answer 22

Phase 3 is Rapid repolarization

Corresponds to the T-wave on EKG

Permeability to NA and Ca stops, and the cell is permeability to potassium

There is a brief relative refractory period until Fully repolarized

Fully repolarized = down stroke of T-wave

Question 23

_{FAST RESPONSE ACTION POTENTIAL(SODIUM MEDIATED)}

Describe Phase 4 of the Fast Response or Sodium Mediated Action Potential:

Answer 23

Phase 4: Resting membrane potential (RMP)

Corresponds to the P-R interval

Maintained by the Na-K pump

Remains at -90 mV until a stimulus comes

Question 24

_{SLOW RESPONSE ACTION POTENTIALS (CALCIUM MEDIATED)}

Which ions primarily mediated the slow action potentials?

Answer 24

Calcium

Question 25

_{SLOW RESPONSE ACTION POTENTIALS (CALCIUM MEDIATED)​}

The slow action potentials are primarily mediated by calcium, but why are they also called calcium-sodium channels?

Answer 25

Because they allow the influx of sodium when they open

Question 26

_{SLOW RESPONSE ACTION POTENTIALS (CALCIUM MEDIATED)​}

What is Phase 0-2? How is it achieved?

Answer 26

Phase 0-2 is Depolarization

It is achieved primarily by the influx of calcium through the slow channels

Allows Na+ to come in with it

• AP develops more slowly and only reaches 0mV (as opposed to +35)*
• Starts less negative (-65) so threshold is achieve easier*

Question 27

_{SLOW RESPONSE ACTION POTENTIALS (CALCIUM MEDIATED)​}

Describe Phase 3

Answer 27

Phase 3 is still repolarization

Still due to K+ exiting the cell

Question 28

_{SLOW RESPONSE ACTION POTENTIALS (CALCIUM MEDIATED)​}

Describe Phase 4

Answer 28

Phase 4: is resting membrane potential

Corresponds to the P-R interval

The membrane is not steady, but rather increases (becomes less polarized/negative)

It increases slowly towards threshold due to a steady decline of the amount of potassium coming out of cell

Na and calcium begins to leak in. And once it reaches threshold, an action potential ensues even without a stimulus

This is known as Spontaneous Deopalarization and account for Automaticity

• RMP starts at only -50 to -65 as opposed to -90*
• Rate of spontaneous depolarization decreases as you descend from SA node (80-100) to the AV node (40-60)*

Question 29

_{SLOW vs FAST ACTION POTENTIALS}

What’s the Biggest difference between slow and fast APs?

Answer 29

Phase 4

In the Fast AP, the RMP stays steady d/t the Na+/K+ pump

In the Slow AP, it spontaneously depolarizes

Also, Slow channels do not have a plateau, and the slow AP starts closer to -65mV, so it gets up the threshold as lot easier than the fast AP which starts at -90mV

Question 30

_{REFRACTORY PERIODS}

What are the two types of

Answer 30

Absolute

Relative

Question 31

_{REFRACTORY PERIODS}

The timeframe where a stronger than normal stimulus cannot cause another AP in an excitable cell while it is depolarized is known as:

Answer 31

An absolute refractory

Shortly after the AP is initiated, the Na channels are inactivated until the membrane returns to the RMP

Na channels remain inactivated until RMP is reached

While the sodium gate is closed, a 2nd stimulus cannot cause depolarization

Question 32

_{REFRACTORY PERIODS}

The timeframe where a stronger than normal stimulus can cause depolarization as that potential reaches back down to RMP is known as:

Answer 32

The relative refractory phase

As the potential returns closer to baseline, a stronger than normal stimulus can cause depolarization.

Causes R on T phenomena

Question 33

_{TRANSLATE INTO PRACTICE}

How does hypokalemia affect RMP?

Answer 33

The lower potassium levels cause hyperpolarization (makes it more negative) of the RMP

Hypokalemia lowers RMP away from threshold

A greater than normal stimulus must come to push the RMP back to threshold so an AP can occur

Question 34

_{TRANSLATE INTO PRACTICE}

How does hyperkalemia affect RMP?

Answer 34

Hyperkalemia raises the RMP (normally -90mV) closer to threshold

Cells fire more easily hence the reason hyperkalemia causes arrhythmias

When resting potential reaches threshold, Na+ gates open and won’t close

For instance normally, the RMP is -90 and threshold is -65, the difference between the two would be 25

In the presence of hyperkalemia if the RMP increases to -70, the difference between the two becomes 15

A less strong stimulus can come in to cause depolarization

Question 35

_{TRANSLATE INTO PRACTICE}

Although calcium has not effect on extracellular K+, why is it often time used as the first line “treatment” for hyperkalemia?

Answer 35

Calcium restores the threshold, shifting it towards a more positive value thus farther away from the RMP that has been reset by the hyperkalemia

So the myocyte excitability can return to normal